This invention relates generally to infrared radiation detection apparatus, and more particularly, to an apparatus and method for remotely detecting the presence of chemical warfare nerve agents in an air-released thermal cloud by analyzing its infrared radiation emission characteristics.
Certain potential enemies of the U.S. possess the capability to direct air-releases of a wide spectrum of lethal and incapacitating chemical warfare nerve agents against the U.S. Fleet. Air-releases of chemical warfare nerve agents, forming thermal clouds, may be effected by dispersal from aircraft or air-burst projectiles. These chemical warfare nerve agents are all organo-phosphorus compounds with strong, narrow infrared absorption bands near 9.8 and 10.75 microns. An interrogation of the infrared characteristics of an air-released thermal cloud must readily permit the detection of chemical warfare nerve agent constituents so that the Fleet may undertake effective countermeasures. The detection of chemical warfare nerve agents must be equally effective in a diurnal or nocturnal environment. The detection of chemical warfare nerve agent thermal clouds must be possible against the varied infrared backgrounds and naturally occuring infrared-emitting objects of the naval environment. The interrogation process must be sensitive enough to discriminate between chemical warfare nerve agent thermal clouds and interferent thermal clouds, i.e., those dispersants which mimic the visual characteristics of a chemical warfare nerve agent thermal cloud such as air-releases of JP-4 aviation fuel and the explosion products from the air-burst of high-explosive projectiles.
The prior art discloses gas analyzers which use infrared radiation to ascertain the chemical composition of a given gas sample; representative examples being U.S. Pat. Nos. 4,297,579 to T. Spaeth, 4,035,643 to J. Barrett, and 4,013,260 to McClatchie et al. These analyzers operate on the principle that a given gaseous element or compound will absorb infrared radiation at specific absorption bands characteristic of that element or compound. Typically, these devices use an active radiation source to generate infrared radiation which is then filtered so as to pass only infrared radiation in a specific absorption band, this absorption band being characteristic of one of the gaseous elements or compounds presumed to be a constituent of the gas sample. The filtered infrared radiation is then passed through a cell containing the gas sample and impinges upon an infrared radiation detection means. The detected infrared radiation is converted into an electrical output signal by means of electronic processing circuitry, the strength of the output signal being inversely proportional to the degree of absorption of the filtered infrared radiation passed through the gas sample. Variations in strength of the output signals at selected absorption bands may then be compared with reference signals generated by passing the same selected infrared radiation bands through the cell without the gas sample to determine the gaseous elements or compounds present in the gas sample. These gas analyzers are active, i.e., to operate they require an infrared radiation source as a component of the apparatus, and the gas sample to be analyzed must be contained within a cell in the apparatus. In contradistinction, a chemical warfare nerve agent detection apparatus, to be efficacious in a naval environment during a conflict scenario, must be a passive device, i.e., function based upon the infrared radiation emissivity characteristics of the chemical warfare nerve agents rather than their absorption characteristics, and must be capable of detecting chemical warfare nerve agents at remote distances.
U.S Pat. No. 2,930,893 to Carpenter et al discloses an apparatus and method for the remote detection of atmospheric contaminants, particularly highly toxic chemical warfare nerve agents. An infrared radiation source-receiver is encircled by a series of remotely positioned reflectors such that coded infrared radiation is transmitted through a detection area and then reflected back through the detection area to the receiver. An echelette grating in the receiver passes infrared radiation at selected infrared wavelengths; a detection band of mean wavelength of 9.8 microns at which the contaminant of interest exhibits strong absorption and reference bands at mean wavelengths of 9.25 and 10.4 microns to reduce the likelihood of any other contaminant in the detection area triggering a false alarm. The received infrared radiation is processed electronically so that a comparator circuit will generate an alarm whenever the toxic agent of interest is present in the detection area. This invention requires an active source of infrared radiation for the detection of chemical warfare nerve agents which detracts from its applicability in a mobile naval environment. Operationally this invention was designed for the detection of chemical warfare nerve agents over a fixed detection area inasmuch as the infrared radiation source-receiver must be encircled by fixed reflectors; this would reduce the invention's efficacy in a mobile naval environment. In addition, the invention's detection area is limited to the line-of-sight between the infrared radiation source-receiver and the fixed reflectors.
U.S. Pat. No. 3,848,129 to Figier et al is representative of passive radiation detection apparatus which is capable of discriminating between infrared radiation with different spectral characteristics. An optical device collects the incident infrared radiation which is then sequentially filtered by first and second bandpass spectral filters to pass infrared radiation of relatively long and short wavelengths. The filtered infrared radiation impinges upon a single detector to produce first and second sampling signals. The first and second sampling signals are compared to produce a comparison signal. A target signal is generated whenever the amplitude of the comparison signal is equal to or greater than a predetermined value. This type of infrared radiation detection apparatus is used to detect point sources of infrared radiation by processing infrared radiation emissions in two narrow spectral bands; the spectral bandpass for potential targets is in the 2.8 to 3.2 micron range while the spectral bandpass for point source background radiation is posited to be in the 2.0 to 2.5 micron range. Therefore this device functions on an either/or basis; intercepted infrared radiation emissions indicate either a true target or a false target. Chemical warfare nerve agents emit infrared radiation in two narrow spectral bands centered at 9.8 and 10.75 microns. In the naval environment, however, most naturally occuring objects have infrared radiation emission characteristics that change slowly with wavelength, i.e., their emissivity over the 8.0 to 14.0 micron atmospheric window is well approximated by that of a gray body; at any given time these objects, as well as interferents, could be emitting infrared radiation in the 9.8 and 10.75 spectral bands. Thus an either/or device would be incapable of discriminating between chemical warfare nerve agents and naturally occuring objects and/or interferents when their spectral characteristics are similar. Finally, an either/or device generates a target signal only when the intercepted infrared radiation generates a comparison signal that exceeds an internal reference; this presupposes a priori knowledge of the signal strength of a target. By comparison, the strength of the signal generated by chemical warfare nerve agents is dependant upon the apparent temperature difference between the chemical warfare nerve agent thermal cloud and its background, the agent concentration in the cloud, and the optical path through the agent cloud; these parameters vary with the physical, chemical and dispersal characteristics of chemical warfare nerve agents and the prevailing atmospheric conditions, and therefore, cannnot be postulated beforehand.
Infrared imaging devices, such as Texas Instruments Model AN/AAS-28, a forward looking infrared (FLIR) thermal-imagery sensor, convert an invisible infrared image into a two-dimensional video image, i.e., a thermogram. Infrared imaging devices are designed to operate within broad infrared wavelength regions of atmospheric transparency, the atmospheric windows. These atmospheric windows exist at approximately 8 to 14 microns, 3 to 5 microns, 2 to 2.5 microns, 1.5 to 1.9 microns and wavelengths shorter than 1.4 micron; infrared radiation emitted at these wavebands by a remote target undergoes minimum attenuation due to atmospheric absorption, scattering and particles prior to reception by a FLIR. A FLIR collects and collimates infrared radiation emitted by a remote target and passes it to infrared detectors which convert the infrared radiation into electrical signals which are amplified and processed by electronic circuitry for real-time viewing on a cathode-ray tube (CRT) or storage on hard copy. Infrared imaging devices spectrally integrate infrared emissions over a broad waveband to form a thermogram which is a composite of all infrared radiation emitted by a remote target. These basic imaging devices do not have the sensitivity required to spectrally discriminate between chemical warfare nerve agent thermal clouds and interferents and/or naturally occuring objects in the naval environment.